X-ray quasars and the X-ray background

The Einstein X-ray observations of a sample of 202 radio- and optically-selected quasars due to Ku, Helfand & Lucy and to Zamorani et al. are analysed. Correlations between X-ray, optical and radio luminosities are examined. The contribution of radio-loud quasars to the 2-keV X-ray background is estimated using high-frequency radio-source counts, and the contribution due to radio-quiet, optically bright quasars using optical counts. It is shown that radio-loud quasars and radio-quiet optically bright quasars together contribute ∼ 15 per cent of the observed 2-keV X-ray background. The contribution of optically faint radio-quiet quasars is uncertain, but may be limited to a maximum of ∼ 30 per cent if recent indications of a flattening in optical counts at faint magnitudes are correct.